Use unicode literals

It's needed for __str__ method properly handle unicode.
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/group_acl.py

@@ -20,6 +20,8 @@
 
 """Group ACL."""
 
+from __future__ import unicode_literals
+
 from django.db import models
 from django.core.exceptions import ValidationError
 from django.utils.encoding import python_2_unicode_compatible